Geostationary satellites are currently in circular orbits on the plane of the earth's equator. Satellite communications using the satellites are very popular. Imagine a sector of a circle with the arc matching a part of the circular orbit of a geostationary satellite and the center matching a location on the earth. The satellite is accessible from the ground location if the satellite is in the part of the orbit (arc) whose center angle is about from 50° to 60°. In addition, adjacent geostationary satellites need to be separated by a distance of about 2° or more in terms of the center angle to avoid interference.
To accommodate a greater satellite communications traffic in the tight satellite orbit space, technological development for reuse of frequency has never been more important than now. One of effective techniques for reuse of frequency in satellite communications is frequency superimposition.
Frequency superimposition in a VSAT (very small-aperture terminal) system will be described in reference to (a) of FIG. 7 to (d) of FIG. 7. The communications system in (a) of FIG. 7 and (b) of FIG. 7 is a VSAT system called P-MP or star network. Satellite communications are performed in the communications system between a single hub station (base station) and multiple very small-aperture terminals (user stations) via a geostationary satellite.
Referring to (a) of FIG. 7 depicting transmissions, generally, the hub station transmits carrier signals with a wide frequency band and the very small-aperture terminals transmit carrier signals with a narrow frequency band. The carrier signal with a wide frequency band transmitted from the hub station is termed outbound signal. The carrier signals with a narrow frequency band transmitted from the very small-aperture terminals to the hub station are termed inbound signals.
An outbound signal generally includes high speed TDM (high speed time division multiplex) channels. Inbound signals in many cases include FDMA (frequency division multiple access) multiplex, narrow bandwidth channels from each very small-aperture terminal or each group of very small-aperture terminals.
The upside-down figure-U symbols in (a) and (b) of FIG. 7 are representations of the spectra of these carrier signals. The greater the size of the symbol is, the wider the frequency band of the carrier signal is. Referring to (b) of FIG. 7 illustrating reception, the hub station and the very small-aperture terminals each receive all of the carrier signals transmitted from that station/terminal and those transmitted from the other stations/terminals. For example, the hub station receives all of the carrier signals (outbound signal) transmitted from the station itself and carrier signals (inbound signals) transmitted from the very small-aperture terminals.
Next will be described the relationships of these carrier signals in frequency bands of the satellite communications channels in reference to (c) and (d) of FIG. 7. (c) of FIG. 7 depicts a frequency relationship of the carrier signals in a conventional communications method. (d) of FIG. 7 depicts a frequency relationship of the carrier signals in frequency superimposition.
In the conventional communications system, the carrier signals for an outbound signal and inbound signals are positioned side by side in separated frequency bands to avoid interference as shown in (c) of FIG. 7. On the other hand, in frequency superimposition, the inbound signals are positioned side by side in the same frequency band as the outbound signal as shown in (d) of FIG. 7. The frequency superimposition theoretically doubles efficiency in the use of the frequency bands of the satellite communications channels.
However, for example, the hub station needs the inbound signals, but not the outbound signal transmitted from itself, because the outbound signal and the inbound signals are superimposed in the same bandwidth in the frequency superimposition.
Therefore, for the hub station to obtain necessary inbound signals, the hub station needs to cancel the unwanted outbound signal to avoid interference between the frequency superimposed outbound signal and inbound signals. A device cancelling such unwanted signals from incoming signals is called a canceller.
In P-MP systems, the power density of the outbound signal transmitted from the hub station to the very small-aperture terminals is greater than the power density of the inbound signals transmitted from the very small-aperture terminals to the hub station due to difference in receiving capability between the hub station and the very small-aperture terminals. The hub station therefore needs a canceller, whereas the very small-aperture terminals do not need on.
The hub station needs to generate a replica of the signal sent from the satellite and subtract the replica to cancel the unwanted carrier in frequency superimposition. There are several ways to generate the replica signal. A first method is to measure the time it takes for the signal to make a round trip from the hub station to the communications satellite and back (“delay time”) to acquire synchronization of the replica signal and the incoming signal. See non-patent literatures 1 and 2.
A second method is to estimate the delay time from real time information on the orbital position of the communications satellite in space. See non-patent literature 3. A third method is to demodulate incoming waves for the unwanted carrier. See (non-patent literature 4.
The first method requires accurate and stable delay time measurement, which would be a problem.
The second method will have trouble in determining the real time orbital position of the communications satellite.
The third method also has a problem that symbol errors could occur in the demodulation for the unwanted carrier. Non-patent literature 4, in relation to the third method of generating a replica signal for cancelling the unwanted carrier, discloses utilization of a difference in power density between the outbound signal and the inbound signals. The method requires no measurement of delay time and is effective when the difference in power density between the wanted carriers and the unwanted carrier is sufficiently large.
In contrast, when the difference in power density between the wanted carriers and the unwanted carrier is not sufficiently large, the measuring of the delay time based on the round trip time to and from the satellite communications is the only method of generating an accurate replica. The present invention assumes this situation. The following will describe a replica signal generation method based on the first method which is applicable to a wide variety of satellite communications lines. In the first method, the delay time is measured by transmitting sum of a primary signal (transmission data) and a wave modulated by a code signal which has a much lower level than the primary signal.
Now, the operation principles of the canceller in the hub station will be described in reference to FIG. 8. The figure illustrates the canceller generating a replica signal (pseudo outbound signal) for the unwanted outbound signal to obtain necessary inbound signals by cancelling the replica signal in the received carrier signals.
Therefore, the canceller's performance depends on how accurately the replica signal (pseudo outbound signal) reproduces the outbound signal. An ideal replica signal matches the outbound signal in all of synchronization timing, frequency, phase, and signal level. Especially important in the delay time measurement in satellite communications is the generating of the replica signal for unwanted carrier cancellation in accurate sync.
If the replica signal is incomplete as illustrated in FIG. 8, the subtraction produces a remaining signal. The remaining signal is termed the remaining error signal because it is an error from the wanted signal, that is, it interferes with the wanted signal. FIG. 9 illustrates exemplary waveforms of an incoming signal, a replica signal, and a remaining error signal produced due to a carrier phase error and a timing error between the incoming signal and the replica signal. A large remaining error signal would affect the BER (bit error rate) performance of the wanted signal.
Next will be described relationship between tolerable BER performance and carrier phase and timing errors in reference to FIG. 10. FIG. 10 illustrates a relationship between carrier power ratio (1/(C/Ihub)) and timing error with varied carrier phase error in 16 QAM (quadrature amplitude modulation). C/Ihub is a carrier interference ratio.
The tolerable level of BER (10−3) required with the replica signal is achieved in the hatched area in FIG. 10. Restricting the tolerable range to this range, the carrier phase error needs to be about 1° and not in excess of 2°. Regarding the timing error, the phase difference needs to be about 3° and not in excess of 5° (about 1/100 symbol). See non-patent literature 2. The tolerable timing phase error is the required delay time measurement precision.
1° in timing error is equivalent to about 0.3 ns in time when the transmission rate is 10 mega symbols/second. The timing error therefore needs to be suppressed to about 0.9 to 1.5 ns. In other words, the delay time measurement is required to have a nanosecond-order precision.
As mentioned above, an accurate replica signal needs to be generated to remove an unwanted signal from received carrier signals to cancel an unwanted carrier signal. High precision delay time measurement is needed for that purpose.
Conventional delay time measurement often involves code synchronization based on a delay time measurement code signal. The characteristics of the delay time measurement device used in the delay time measurement vary largely depending, among others, on the correlation property of the delay time measurement code signal, the length of the cycle given as an integral multiple of one bit for a code signal (hereinafter “cycle”), and the frequency band width (integration time for a correlator) for the code synchronization circuit in a delay time measurement device.
Some conventional delay time measurement devices involves code synchronization using a single PN (pseudo noise) code signal as the delay time measurement code signal. The PN code signal is also called the pseudo noise signal.
However, to achieve high precision measurement, this delay time measurement method using a single PN code signal needs an extremely long cycle and synchronization of PN code signals with a long cycle. These requirements lead to an undesirable, long measurement time. The conventional delay time measurement device may take about 10 minutes to measure the time, for example. This is not practical.
Meanwhile, interference between the PN code signal and the carrier signal needs to be avoided. The PN code signal level needs to be sufficiently lower than the carrier signal level. Therefore, the transmission/reception of the PN code signal needs to be carried out in a very low C/N (carrier to noise) environment, which is also a problem. Note that C is the signal level of the delay time measurement code signal and N is the signal level of the signal unwanted in receiving the code signal.
The delay time measurement is required to have a nanosecond order measurement precision as mentioned earlier. Conventional methods take too long a time to meet the required measurement precision level in the low C/N environment.
Synchronization of the delay time measurement code signal is typically acquired by using a correlator outputting a correlation level which corresponds to a theoretical correlation value. The theoretical correlation value indicates similarity between the delay time measurement code signal and the sum of the carrier signal and the delay time measurement code signal. The delay time measurement code signal is typically determined to be in sync if the correlation level exceeds a predetermined threshold.
The theoretical correlation value is constant. On the other hand, if the input level of the received carrier signal varies for whatever reason, the correlation level obtained from the correlator varies with the variation. Accordingly, if the predetermined threshold is fixed, synchronization cannot be acquired in a stable manner.